The disclosure relates to the isolation of dianhydrides. In particular, the disclosure relates to the isolation of dianhydrides with low color.
Polymers such as polyetherimide are most desirably colorless or very light colored (have low color). Polyetherimide is typically produced by reacting dianhydrides with diamines. It is therefore desirable that the dianhydrides and diamines have low color in order to produce polyetherimide with low color.
A process to isolate dianhydride from an exchange reaction comprises extracting a bisimide/anhydride exchange reaction aqueous phase with an organic solution comprising an exchange catalyst at a first temperature and pressure to result in an extracted aqueous phase comprising water, exchange catalyst and a dianhydride precursor; removing water from the extracted aqueous phase at a second temperature and pressure to form a molten phase wherein the second pressure is less than the first pressure; removing water and exchange catalyst from the molten phase at a third temperature and pressure to form an isolation mixture; and converting the dianhydride precursor in the isolation mixture to dianhydride at a fourth temperature and pressure, wherein the fourth temperature is greater than the second and third temperatures and the fourth pressure is less than the second and third pressures.
In another aspect, a process to isolate dianhydride from an exchange reaction comprises extracting a bisimide/anhydride exchange reaction aqueous phase with an organic solution comprising an exchange catalyst at a first temperature and pressure to form an extracted aqueous phase comprising water, exchange catalyst and dianhydride precursor; feeding the extracted aqueous phase to a flash vessel operated at a second temperature and pressure to form a molten phase; feeding the molten phase to a falling film evaporator operated at a third temperature and pressure to produce an isolation mixture having an exchange catalyst to dianhydride precursor ratio of about 0.4 to about 0.8; converting the dianhydride precursor in the isolation mixture to dianhydride in a wiped film evaporator at a fourth temperature and pressure.
In another aspect, a system for dianhydride isolation comprises a flash vessel that receives an extracted aqueous phase from an extraction column and forms a molten phase, a falling film evaporator in fluid communication with a first outlet of the flash vessel, wherein the falling film evaporator receives the molten phase from the flash vessel and forms an isolation mixture; and a wiped film evaporator in fluid communication with a first outlet of the falling film evaporator, wherein the wiped film evaporator receives the isolation mixture from the falling film evaporator and comprises a first outlet for collection of the isolated dianhydride.